Frequency really is just familiarity.
When I am familiar with you - I trust you.
If I just met you, I'm less likely to trust you.
This sounds like a simple concept but people get this wrong with ads all the time.
They assume that people should buy the fist time they see the ad. The first time they engage with you but that is hardly ever the case. They need to get to know you and your brand. They need to get familiar, so that they can have enough confidence to give you a try.
While it's not always about familiarity it is always about facilitating some level of trust.